How can I detect when the XAML Slider is Completed?

XAML, WinRT, Windows8.1 and UWP:

PointerCaptureLost event should work for mouse / touch
KeyUp event for keyboard

You can create a new class and inherit from Slider. From there on, you can look for the Thumb control & listen for the events you want.

Something like this should work:

public class SliderValueChangeCompletedEventArgs : RoutedEventArgs
{
    private readonly double _value;

    public double Value { get { return _value; } }

    public SliderValueChangeCompletedEventArgs(double value)
    {
        _value = value;
    }
}
public delegate void SlideValueChangeCompletedEventHandler(object sender, SliderValueChangeCompletedEventArgs args);

public class ExtendedSlider : Slider
{
    public event SlideValueChangeCompletedEventHandler ValueChangeCompleted;
    private bool _dragging = false;

    protected void OnValueChangeCompleted(double value)
    {
        if (ValueChangeCompleted != null)
        {
            ValueChangeCompleted(this, new SliderValueChangeCompletedEventArgs(value) );
        }
    }

    protected override void OnApplyTemplate()
    {
        base.OnApplyTemplate();
        var thumb = base.GetTemplateChild("HorizontalThumb") as Thumb;
        if (thumb != null)
        {
            thumb.DragStarted += ThumbOnDragStarted;
            thumb.DragCompleted += ThumbOnDragCompleted;
        }
        thumb = base.GetTemplateChild("VerticalThumb") as Thumb;
        if (thumb != null)
        {
            thumb.DragStarted += ThumbOnDragStarted;
            thumb.DragCompleted += ThumbOnDragCompleted;
        }
    }

    private void ThumbOnDragCompleted(object sender, DragCompletedEventArgs e)
    {
        _dragging = false;
        OnValueChangeCompleted(this.Value);
    }

    private void ThumbOnDragStarted(object sender, DragStartedEventArgs e)
    {
        _dragging = true;
    }

    protected override void OnValueChanged(double oldValue, double newValue)
    {
        base.OnValueChanged(oldValue, newValue);
        if (!_dragging)
        {
            OnValueChangeCompleted(newValue);
        }
    }
}

You can use pair of bool values isValueChanged and (if possible change value without manipulation of pointer ) isPressed;

private void Slider_ValueChanged(object s, RangeBaseValueChangedEventArgs e) {
    if (!isPressed) {
        AcceptChanges();
    } else {
        isValueChanged = true;
    }
}

Initialization code:

Window.Current.CoreWindow.PointerPressed += (e, a) => { isPressed = true; };

Window.Current.CoreWindow.PointerReleased += (e, a) => {
    isPressed = false;
    if (isValueChanged) AcceptChanges();
};